The human iris is a muscle for controlling pupil dilation and consequently how much light enters the eye for image formation on the retina. This muscle has such rich variations in pigmentation patterns across the global population that no two people ever have the same patterns. Even twins and eyes of the same individual are greatly different and this provides an opportunity for powerful identity checking technology. It is a result of the vast number of degrees of freedom inherent in iris patterns compared with the size of the global population, or even compared with the number of humans that have ever or will ever exist, that iris recognition offers such a powerful opportunity for identity verification. Iris recognition technology is much better at differentiating between individuals than traditional methods such as presenting original photo documentation, signing, using chip and PIN and fingerprint matching.
Systems based on iris recognition are applicable in a range of applications including commercial and official contexts where identity checks are important for commercial, legal, security or other reasons. For example, certain airports currently employ the use of iris cameras for verifying the identity of individuals crossing national boarders.
Iris cameras for use in iris recognition technology must obtain images that can be checked against reference images forming part of a user profile for that individual. The iris camera is therefore one element of the whole iris recognition system, where other elements include a database storing reference images, or at least data derived from such images, as part of a remote user profile for registered users of iris recognition. If the user profile contains reference data derived from an initial reference image, the reference data acts like a barcode uniquely identifying that user. This saves on memory required to store the material against which future identity checks are made.
In order to register for iris recognition a user must therefore have reference images of their irises taken for their user profile. An iris camera is clearly required for this registration process, as well as for subsequent instances when the technology is being used by the user to access various rights or authorise a transaction. However, the camera used in the registration step is not necessarily the same camera as the one used in subsequent instances because of course a user may register, for example, at a bank and subsequently require ATM services elsewhere gaining access by iris recognition.
In order to be practically useful, iris cameras must be able to perform well with a high volume of users. This means firstly that the system must be straightforward and convenient to use for each individual using the camera. One of the most important practical considerations in terms of convenience of use is to make it easy for the user to position their head correctly for the camera to acquire images of both irises. Different users will naturally adopt varying positions in the forwards-backwards dimension and the camera system must be able to cope with this, rapidly acquire both irises and be ready to repeat the process for the next user. The depth of the volume in which each of a user's eyes can be positioned for successful iris capture —i.e. the depth of the capture box—should therefore be reasonably generous.
The requirement that the camera must cope with a high volume of users means, secondly, that no classes of iris types or positions should present problems to the camera system. One aspect of this issue is related to iris positions based on the distance between a user's pupils. Certain racial heritages are associated with particularly wide or particularly narrow inter-pupillary distances compared with the majority in the global population. The iris camera's field of view should be adequate to accommodate the full range of inter-pupillary distances with which it might be presented.
In order to provide a deep capture box and a wide field of view, some known methods use iris cameras with high curvature lenses associated with a high depth of field. The high depth of field provides a deep capture box and the high curvature lens surface captures incident light at wide angles. However, in order to mitigate defocusing effects near the edges of the lens which are most pronounced in high curvature lenses, diaphragms are placed in front of the lens the reduce the aperture and effectively refocus the resultant image. This reduces the overall amount of light that can pass through the lens and form an image on the other side. Consequently, image resolution is poor and typically the optical resolution of the image falls short of the equivalent resolution—in pixels per unit area—of the camera's image sensor. In this way, the optics of many known iris cameras degrade the quality of the image which could be obtained by the image sensor.
Other known methods use a lower curvature lens so that defocusing effects at the lens periphery are less and a diaphragm with a larger aperture can be used to let more light in. This improves image resolution. However, the shallow lens curvature tends to restrict field of view, which in turn restricts the width of the capture box, making it inconvenient for users and reducing the range of inter-pupillary distances that can be accommodated. Furthermore, the shallow curvature lens is associated with a small depth of field (shallow focus) which does not extend across the full range of the capture box. In order to produce in-focus images of irises in the capture box, the lens itself must therefore be moved to suit the position of the user's eyes. Automatic fine-focus systems are typically employed having motors controlled by various electronic feedback loops, which move the lens so that the in-focus region corresponds to the position of the irises.
The feedback loops form part of an iterative autofocus system in which successive captured images are digitally analysed to assess their level of focus. Image analysis software provides a measure of focus for successive images, successive measurements of improving or worsening focus are used to make decisions as to where the lens should be moved next. The images gradually approach focus and once an acceptable level of focus has been achieved iris images can be captured.
However, this process is time consuming If the lens starts far away from the in-focus position, the iterative procedure of fine-focus will contribute significantly to the total time from start to finish for a user to position him or herself at the device and for dual iris acquisition to be completed. This time-consuming feature is clearly disadvantageous where the camera is to be used in a high throughput application, for example scanning people's irises as a security measure at an airport, for example.
The present invention seeks to address some or all of the above issues.